The present invention relates to a signal disconnection detection circuit. More particularly, the present invention relates to a signal disconnection detection circuit that is applicable to subscriber line terminals (SLTs) for a digital subscriber line transmission system.
Signal disconnection detection circuits of this type are disclosed in, for example, Japanese Patent Laid-Open No. 3-36848. A reception signal detecting circuit disclosed therein is used to detect the presence of a reception signal in a reception circuit within a line terminal for a digital subscriber line transmission system.
The reception circuit has a waveform equalizer, an A/D converter, a symbol discrimination circuit, and a reception signal detecting circuit. The waveform equalizer is for equalizing a waveform of a reception signal and for shaping a waveform of the reception signal degraded due to transmission path characteristics. The A/D converter is for converting an analog output of the waveform equalizer into a digital signal. The signal discrimination circuit and the reception signal detecting circuit each receives an output of the A/D converter.
The reception signal detecting circuit has a positive comparator, a negative comparator, an AND circuit, and a protection circuit. The positive comparator compares an input signal with a positive threshold value. The negative comparator compares the input signal with a negative threshold value. The AND circuit receives the outputs of the positive and negative comparators.
Operation of the above-mentioned reception signal detecting circuit is described. In this example, a four-value signal is used with the levels of '1 and .+-.3. The output of the A/D converter is supplied to the positive comparator and the negative comparator. The positive comparator compares the input signal with the positive threshold value. The positive comparator then produces a positive detection signal at a high level only when the input signal is smaller than the positive threshold value.
On the other hand, the negative comparator compares the input signal with the negative threshold value. The negative comparator produces a negative detection signal at a high level only when the input signal is larger than the negative threshold value. The positive and negative threshold values herein are determined between a transmission signal level and a zero level. In other words, the positive and negative threshold values are determined to have a value that is smaller than the level range of the .+-.1 level transmission signal and that is larger than a noise level. Comparisons in the positive and negative comparators are performed every time of symbol discrimination.
The protection circuit is for use in avoiding incorrect decision due to, for example, noises on a line. The protection circuit considers, as the detection of a reception signal, a predetermined number of protection stages being reached for the number of detection signals produced during a predetermined period by the positive and negative comparators. In response to the detection, the protection circuit produces a detection signal DET.
An exemplified protection circuit ANDs reproduction clocks that are extracted from the reception signal. The protection circuit counts up the number of times (number of pulses) when both the positive and negative comparators produce the signal of logic "1" during a unit time (such as a single frame of data). When the counted value is equal to or larger than a predetermined number, then the protection circuit determines signal disconnection and produces a signal of logic "1". Otherwise, the protection circuit produces a signal of logic "0".
Reception circuits with a decision feedback equalizer have been proposed in order to improve equalization characteristics and reduce training time in the reception circuit having the above-mentioned configuration. The outputs with different isolated response waveforms are produced from the waveform equalizer with or without the decision feedback equalizer. Without the decision feedback equalizer, the waveform equalizer is required to make the amplitude obtained at all sample points other than a decision point be zero. On the other hand, the waveform equalizer is required to make only the amplitude obtained at sample points before the decision point be zero.
A signal disconnection detection circuit with the decision feedback equalizer is described below. The signal disconnection detection circuit comprises an absolute value circuit, an average value circuit, and a comparator. The reception signal is supplied to the absolute value circuit. An output of the absolute value circuit is supplied to the average value circuit. An output of the average value circuit is then supplied to the comparator. The absolute value circuit takes an absolute value of the reception signal. The average value circuit monitors absolute value signals for individual samples for a predetermined time and calculates an average value level during that period. The comparator compares the average value level with a predetermined threshold value. The comparator determines the presence of a signal when the average value level is larger than the predetermined threshold. The comparator determines the disconnection of the signal when the average value level is smaller than the predetermined threshold.
The average value circuit is formed of a unit delay circuit and an adder. The average value circuit monitors the signals for a period determined by the unit delay circuit to obtain an average of the absolute value signals for the individual samples.
However, the above mentioned signal disconnection detection circuit may sometimes not detect the disconnection of the signal when the decision feedback equalizer is used as the waveform equalizer. The reason is as follows. The decision feedback equalizer estimates intersymbol interference due to post-cursors according to the result of the symbol discrimination. The decision feedback equalizer thus produces an incorrect post-cursor estimation if it receives a signal other than the discrimination signal that is produced upon the disconnection, though the signal is actually disconnected. The signal disconnection detection circuit considers the incorrect estimation as a signal. This means that the decision feedback equalizer doesn't produce a zero output. The signal disconnection detection circuit detects no signal disconnection indefinitely.
The average value circuit requires an arithmetic unit that enlarges the circuit itself. In addition, it takes a relatively long time to detect the signal disconnection after the reception of the signal because of the calculation for an average value during a certain period. Furthermore, detection characteristics for the comparator depend on the predetermined threshold value, so that it is difficult to determine this threshold value.